This invention relates to a press mold that is adapted to press-mold a molding material such as a glass by the use of upper and lower molds applied with precision machining and does not require post-processing such as grinding or polishing with respect to a molded surface, and further relates to an optical element manufacturing method using such a press mold.
A method is known where a molding material such as a glass is heated to be softened and then press-molded by a pair of upper and lower molds precisely machined into predetermined shapes, thereby manufacturing an optical element such as a lens [see, e.g. Patent Document 1 (Japanese Patent (JP-B) No. 3501580) and Patent Document 2 (Japanese Unexamined Patent Application Publication (JP-A) No. H09-286622)].
Patent Document 1 describes a molding method where a pair of positioning members are moved in a press mold comprising upper and lower molds and are brought into contact with an optical material (molding material) in a sandwich manner, thereby positioning the optical material with respect to the press mold. It is described that, particularly, when molding a double-sided concave lens, the optical material is placed on the lower mold having a convex shape and, therefore, if it is placed in an offset manner, there is a possibility of slipping off the lower mold and thus the positioning of the optical material is required in the press mold.
Patent Document 2 describes a method where a glass preform (molding material) is held by holding means at a position spaced apart from a press mold comprising upper and lower molds, then heated, released from the holding means, and then pressed. It is described that, with this configuration, a chemical reaction between the glass preform and the press mold can be avoided during heating and molding can be carried out without impeding the flow of the glass preform in its radial direction during pressing.
When forming a molding material (glass material etc.) into an optical element such as a lens by precision press molding, it is general that the molding material is press-molded between a pair of upper and lower molds having opposed molding surfaces. In this event, it is necessary to supply and place the molding material on the molding surface of the lower mold in advance, but, depending on the shape of the optical element to be obtained, it is not necessarily easy to place the molding material at the center position of the molding surface of the lower mold.
As such an example, there can be cited a case of, for example, supplying and placing the molding material on the molding surface of the lower mold having a convex surface, such as a case of molding a both-sided concave lens. Other than this, positioning of the molding material is difficult when there is no concave surface having a proper curvature at the center area of the molding surface of the lower mold (when the center area of the molding surface of the lower mold is a convex or flat surface).
In those cases, when, for example, the molding material placed on the molding surface of the lower mold is offset in position or slips off at the time of press molding, thickness deviation occurs in the optical material to be molded so that not only shape failure is resulted, but also surface accuracy in terms of the optical function is degraded due to unevenness in load application caused by the thickness deviation.
Further, when manufacturing an optical element by transferring a press mold containing a molding material therein to a plurality of process chambers for heating, pressing, cooling, etc. in sequence (details will be described later), the molding material contained in the press mold is subjected to position offset caused by vibration etc. of the press mold during transfer to the respective chambers (particularly upon starting or stopping) and, if molding is carried out while the molding material is offset in position, thickness deviation occurs in the optical material, thus resulting in shape failure.
According to the description of Patent Document 1, the positioning members for the optical material are disposed in the press mold and these positioning members are moved in mutually opposite directions with respect to the reference position by the use of rack-and-pinion drive means and stopped when brought in contact with the optical material in the sandwich manner, so that positioning of the optical material is carried out with respect to the press mold. The positioning members are retreated by the drive means when or immediately before molding surfaces of the upper and lower molds contact the optical material at the time of pressing.
According to this method, however, since the positioning members are disposed inside the press mold, the structure of the press mold becomes quite complicated. Consequently, the heat capacity of the press mold increases so that it becomes difficult to efficiently execute a control of temperature rise and drop. Further, when the structure like the rack-and-pinion drive means is disposed near the press mold, not only a press molding machine increases in size, but also necessity arises to consider the influence of thermal deformation of the structure and so on, so that a machine design is extremely complicated.
When a press mold comprising upper and lower molds is fixed in a press molding machine to thereby carry out heating, pressing, and cooling at the same position, it is possible to some degree to perform positioning of a molding material by the use of the foregoing movable members that cause the complication of the machine. However, when manufacturing an optical element by placing a molding material in a press mold separated from a press molding machine and applying proper processes to the press mold in sequence while transferring the press mold in the machine, it is extremely inefficient to provide the foregoing elaborate movable members for each of individual press molds, which is practically impossible.
Patent Document 2 shows a drawing where a flat-plate preform is press-molded by upper and lower molds each having a convex surface. Specifically, heating is carried out in the state where the preform is placed at an upper end of a holding ring, then the holding ring is moved downward by drive means so as to place the preform on the lower mold, and then the preform is pressed between the upper and lower molds. In this method, since the preform is in constant contact with the inner periphery of a lower sleeve, position offset of the preform does not appear to occur even when the molding surface of the lower mold has the convex shape.
However, since there is no means for regulating the horizontal relative position between the upper and lower molds, the coaxiality between the upper and lower molds cannot be achieved. As a result, there occurs eccentricity between the first plane and the second plane (shift between them in the horizontal direction and tilt between them) of a molded optical element so that the sufficiently high optical performance cannot be obtained.